/Engine/Source/Runtime/InteractiveToolsFramework/Public/BaseGizmos/GizmoMath.h
Project the input Point into the plane defined by PlaneOrigin and PlaneNormal, and then measure it's signed angle relative to the in-plane X and Y axes defined by PlaneAxis1 and PlaneAxis2
the signed angle
namespace GizmoMath { float GizmoMath::ComputeAngleInPlane ( const FVector & Point, const FVector & PlaneOrigin, const FVector & PlaneNormal, const FVector & PlaneAxis1, const FVector & PlaneAxis2 ) }